The present invention relates to a new and improved construction of a passive picture-resolving detector assembly.
In its more specific aspects, the present invention particularly relates to a new and improved construction of a passive picture-resolving detector assembly comprising a matrix detector and imaging optical means for forming an image of an object scene on the matrix detector. The matrix detector includes a two-dimensional array of detector elements each of which receives, in cyclic succession, a plurality of picture elements of such image in order to improve resolution.
Such picture-resolving detector assemblies are used, for example, for steering target tracking missiles towards a detected target or in end phase-steered munition. The matrix detector responds to infrared radiation which emanates from a target within an object scene. The matrix detector provides a raster of picture elements (pixels). These picture elements are subjected to picture processing in order to detect and identify a target and to determine the target deviation from the center of the field of view of the detector assembly. In order to reduce noise to an acceptable value, the matrix detector must be cooled down to a very low temperature by means of a cooler.
It is desirable, on the one hand, to cover a field of view as large as possible by the matrix detector in order to also detect targets having rather large target deviation. On the other hand, a geometric resolution as high as possible is required. Also, if the distance to the target is large, adequate resolution of the target structures must be possible. Thereby, identification of the target is to be facilitated and the false alarm rate, i.e. the probability of false classification of the target, is to be reduced.
In the infrared wavelength ranges of 3 to 5 .mu.m and 8 to 12 .mu.m used here, optical systems can be designed to be diffraction limited. Therefore, the optical systems provide high-resolution picture information. It presents, however, problems to construct a matrix detector which is capable of transferring this high-resolution picture information with least possible loss to the picture processing system. The quality of the system is determined by the modulation transfer function of the detector elements of the matrix detector. The modulation transfer function represents the capability of the detector elements of the matrix detector to scan structural information without deterioration.
Ideally, very large detector arrays having, for example, 1024 .times.1024 detector elements should be used in order to take advantage of the full power of the imaging optical system. Such detectors, however, are not available.
From, for example, European Patent No. 0,133,890, granted Jan. 11, 1989, there is known an electro-optical detector system for generating electronic picture information using a charge coupled matrix detector which consists of an array of rows and columns of square detector elements. Interstices between the detector elements have a width which substantially corresponds to the side lengths of the detector elements. By means of a wobbling mirror, which can be advanced successively to four positions and which reflects the imaging path of rays, an image of an object scene is stepwisely moved relative to the matrix detector along a square trajectory. Thereby, each detector element detects, in cyclic succession, four adjacent areas of the image of the object scene. In a computer, the picture information is then composed to form a continuous "picture". By means of this image shift the resolution of the matrix detector is improved which resolution otherwise would be limited by the spacing between the detector elements. Using such structure of the detector assembly, a picture sensor of effectively 1024.times.1024 detector elements could be realized with no more than 256.times.256 detector elements. This would be within the range of the technologically possible.
At a side length of 20 .mu.m which is the limit of what can be technologically accomplished, such matrix detector would result in a detector chip of at least 30 mm diameter. The cooling of such detector chip down to typically 80 to 100K in a missile seeker head is very expensive when employing available cooling techniques. In addition, the matrix detector would have to be manufactured from individual detector elements. This is uneconomic and expensive.
"Micro-mechanical mirror arrays" are known. Such micro-mechanical mirror arrays consist of a multitude of contiguous mirror elements. The mirror elements have a size of typically 20.times.20 .mu.m. Using binary control, the mirror elements can be moved into a respective one of two operative positions. These mirror arrays are made by means of silicon chip technology. One construction of such micro-mechanical mirror arrays is commercially available from Texas Instruments Inc. and called "Digital Micromirror Device DMD".